Please help! GFX card problem that I can't wrap my head around!

Alright, so. I bought a new pc a little over a year ago. Since the day I bought it, it has hard crashed randomly when playing certain games. For example, it hard crashes the computer when I play crysis, or sacred 2, or stalker...but it does not crash when I play any HL2 mods or games, tf2, bioshock 2.

...The card and machine are perfectly clean and are not overheating (running temp is approximately 60
I have no registry issues, viruses, adware/spyware etc. I've RMA'd this card, it was verified broken and they sent me a new one...same issues occur.

Here are my specs,

Windows: Microsoft Windows 7 6.1.7600
Internet Explorer: 8.0.7600.16385
Memory (RAM): 8192 MB
CPU Info: Intel(R) Core(TM)2 Quad CPU Q8300 @ 2.50GHz
CPU Speed: 2497.9 MHz
Sound card: Speakers (Realtek High Definiti
Display Adapters: ATI Radeon HD 4850 | ATI Radeon HD 4850 | RDPDD Chained DD | RDP Encoder Mirror Driver | RDP Reflector Display Driver
Monitors: 1
Screen Resolution: 1280 X 1024 - 32 bit
Network: Network Present
Network Adapters: Atheros AR8121/AR8113/AR8114 PCI-E Ethernet Controller(NDIS6.20)
CD / DVD Drives: F: TSSTcorpCDDVDW SH-S223F
COM Ports: COM1
LPT Ports: NOT Present
Mouse: 8 Button Wheel Mouse Present
Hard Disks: C: 465.8GB | D: 74.5GB | E: 74.6GB
Hard Disks - Free: C: 351.7GB | D: 66.4GB | E: 32.5GB
USB Controllers: 8 host controllers.
Firewire (1394): 1 host controllers.
PCMCIA (Laptops): Not Installed
Manufacturer: American Megatrends Inc.
Product Make: System Product Name
AC Power Status: OnLine
BIOS Info:
Time Zone: Pacific Standard Time
Battery: No Battery
Motherboard: ASUSTeK Computer INC. P5Q-PRO
Modem: Not detected
:

I am at my wits end. The power supply is more than adequate which is the only other issue I can think of...some graphic intensive games run fine while others don't. I've rma'd it already...what could be going wrong?

The company I bought it from (NCIX) has been less than helpful and accommodating in regards to fixing my problem so, I am thinking of simply switching cards to Nvidia...likely a palit 260GTXsonic. Instead of dealing with them further...would this definitely fix my problem? pls help!

Thoughts? is there anything else I can do to narrow this problem down further?

Thanks very much for taking a look folks, any and all help is greatly appreciated.